Proportional to absolute temperature (PTAT) bias generators are well known. In general, PTAT generators have two stable states--a desired state and a state in which all the currents are zero. To ensure that the PTAT generator has the desired state when the currents are greater than zero, a separate start-up circuit is provided. An example of a known PTAT generator is shown in FIG. 1 and will now be described.
The PTAT generator is referenced by reference numeral 2. The PTAT generator 2 has a first pair of P channel field effect transistors (FETs) formed by a first P channel transistor 6 and a second P channel transistor 8. The first pair of transistors 6 and 8 are a matched pair. In other words, the two P channel transistors have the same general characteristics. The gates 10 and 12 of the first pair of transistors 6 and 8 are connected. The gate 10 and drain d of the first P channel transistor 6 are connected to each other. The sources s of the first and second transistors 6 and 8 are connected to a voltage supply.
The PTAT generator 2 has a second pair of N channel transistors including a third N channel transistor 16 and a fourth N channel transistor 18. The second pair of transistors 16 and 18 are also a matched pair of transistors. The drain d of the third N channel transistor 16 is connected to the drain d of the first P channel transistor 6. Likewise, the drain d of the fourth N channel transistor 18 is connected to the drain d of the second P channel transistor 8. The gates 20 and 22 of the third and fourth N channel transistors 16 and 18 are connected to each other. The drain d and gate 22 of the fourth transistor 18 are connected together.
A first diode 28 is connected between the source s of the third N channel transistor 16 and ground. The source s of the fourth transistor 18 is connected to one end of a resistor 32. A second diode 30 is connected between the other end of the resistor 32 and ground. The first diode 28 is relatively small while the second diode 30 is relatively large. A start-up current from a start-up circuit (not shown) is applied via node 42. The bias current output from the PTAT generator is taken from node 27, which is between the drain d of the second P channel transistor and the drain d of the fourth N channel transistor 18.
The operation of the PTAT generator 2 will now be described. If no start-up current is applied to the PTAT generator 2 when the voltage supply is turned on, the PTAT circuit may be in an unknown state and not in a desired stable state. In use, the PTAT generator 2 should be in the stable state when current flows. In the case when no start-up current is applied, the initial voltage at node 42 effectively will determine the state of the PTAT generator 2. The voltage at node 42 can initially be high, low or in between these values.
If the voltage at node 42 is initially high, then the first pair of P channel transistors 6 and 8 will be off as a high voltage is applied to their gates 10 and 12. This in turn means that, as the second transistor 8 is off, a low voltage is applied to the gates 20 and 22 of the third and fourth N channel transistors 16 and 18. These two N channel transistors 16 and 18 will therefore also be off. Accordingly, the PTAT generator 2 is in a state where no current flows.
However, if the voltage at node 42 is initially low, then a low voltage is applied to the gates 10 and 12 of the first and second P channel transistors 6 and 8. These two transistors 6 and 8 are therefore turned on. As the second transistor 8 is on, there will be a voltage applied to the gates 20 and 22 of the third and fourth N channel transistors 16 and 18. The third and fourth N channel transistors 16 and 18 are therefore both on. A current will therefore flow through the first transistor 6, the third transistor 16 and the first diode 28. Likewise a current will flow through the second transistor 8, the fourth transistor 18, the resistor 32 and the second diode 30. The PTAT generator 2 will therefore be in a stable state when current flows.
The desired stable state, the four transistors 6, 8, 16 and 18 will all be conducting at the same time. If the voltage at node 42 is initially neither low nor high, the state of the PTAT generator is difficult to predict, and some current may be flowing. The application of a start-up current to node 42 ensures that the PTAT generator will be in the stable state when current flows. The current which is applied is such that the node 42 is momentarily pulled low, regardless of the initial voltage at this node 42 when the voltage supply is first applied to the PTAT generator 2.
The resistance of the resistor 32 will depend on the temperature, and as the current provided by the PTAT generator 2 is related to the resistance of the resistor 32, compensation for variations in temperature will be provided. As the first diode 28 is relatively small compared to the second diode 30, this compensates for the presence of the resistor 32 so that the current on each side of the PTAT generator 2 can reach an equilibrium. An equilibrium state is achieved when the current on each side of the PTAT generator 2 is the same. The arrangement of a resistor and a large diode matching a small diode reaches equilibrium such that current flowing is proportional to absolute temperature.
In conventional analog designs, this start-up current will be provided by a fixed current source, such as a permanently conducting MOSFET or JFET device providing a weak current. The start-up current is provided until the current provided by the PTAT circuit exceeds that from the start-up current source. However, this type of start-up circuit is disadvantageous in that either the start-up circuit provides a large current which consumes excessive power, or the start-up circuit provides a small current which requires a physically large device which is wasteful of space, particularly if the start-up circuit and PTAT generator are part of an integrated circuit.